Transformer coupled multi vibrator pulse generator for fault detection

ABSTRACT

An AC pulse generator for use in line fault detection is constructed as a manually portable transistorized multivibrator for producing a distinctive signal having a pip, which is applied to the circuit under test. Doing so produces unbalanced currents which produce a net magnetic field along the current path, which can then be detected.

United States Patent [72] Inventor Rudolph J. Sturm, Jr. FOREIGN PATENTS594,311 3/1960 Canada 331/1131 Northfleld, Mas. 55057) 2 Appl 7 0 2 1OTHER REFERENCES [22] Filed Sept. 17,1968 Butler, Transistor Inverterand Rectifier-Filter Units,

Division of Ser. No. 314,727, Oct. 8, 1963, Pa t. N 3,441,842 [451Patented Mar. 23, 1971 [56] References Cited UNITED STATES PATENTS3,008,068 11/1961 Wiltingetal. 33l/ll3.(lX)

ELECTRONIC ENGINEERING, July 1959, pp. 412-418. (331- 1 13.1

Primary Examiner-Roy Lake Assistant Examiner-Siegfried H. GrimmAtt0rney-Hill, Sherman, Meroni, Gross and Simpson ABSTRACT: An AC pulsegenerator for use in line fault detection is constructed as a manuallyportable transistorized multivibrator for producing a distinctive signalhaving a pip, which is applied to the circuit under test. Doing soproduces unbalanced currents which produce a net magnetic field alongthe current path, which can then be detected.

TRANSFORMER COIJFLED MULTI VIBRATOR PULSE GENERATOR FOR FAULT DETECTIQNThis application is a division of my copending application Ser. No.314,727, filed Oct. 8, l963now U.S. Pat. No. 3,441,842.

This invention relates to an AC pulse generator for use in detecting thelocation of a short in a shorted circuit.

Various kinds of test equipment have been provided by which the locationof a circuit may be detected. When the circuit under test is locatedphysically adjacent to many other circuits which may then be energized,or when the circuit under test is located on a structure embodying aconsiderable amount of steel, or when both such conditions are presentas in a dieseLeIectric locomotive, the problems involved in finding suchcircuit or short are greatly multiplied. One such type of testequipment, which has been used to locate shorts or unwanted grounds inrailroad locomotives and has a cost near $5,000.00, is mounted on a cartthat needs the power of one man to push it across tracks to thelocomotive to be tested, and contains a transformer which must beconnected to a commercial source of alternating current. Such knownequipment is of relatively low sensitivity, draws about 1 kilowatt ofpower, and may deliver a current at a voltage up to 2,000 volts. Thehigh voltage leads are connected to the faulty electric circuit, whileanother portion of the equipment is carried about by means of a shoulderstrap to enable a rather heavy sensing coil to follow the circuitry tothe fault. I am aware of two fatal accidents that have occurred as adirect result of use of this type of equipment.

The present invention is directed to a small, portable, selfpowered,lightweight source of current which can be housed in a cabinet the sizeof which is about a 5 inch cube and which weighs less than 5 pounds.This current source is safe to both the user and to the equipment beingtested since the power it delivers is usually less than 1 watt. Themeans by which current is applied to the circuit being tested is uniquein that it comprises a composite AC pulse which preferably is rich inharmonics.

Accordingly, it is an object of the present invention to provide a novelsignal source for use in detecting the location of shorts, grounds, orcircuits.

A still further object of the present invention is to provide a signalsource by which the location at which a conductor enclosed within asteel conduit and shorted to such conduit can be identified.

Another object of the present invention is to provide a novel signalsource enabling detecting the location of a short between a pair ofphysically parallel conductors in a shorted circuit.

Yet another object of the present invention is to employ a harmonicallyrich signal source to produce magnetic fields in one conductor which arenot entirely cancelled by the adjacent field of another parallelconductor of the transmission line.

A still further object of the present invention is to provide an ACpulse generator.

Another object of the present invention is to provide testing means ofthe type described which can be employed while adjacent circuitry isleft energized.

Another object of the present invention is to provide a pulse generatorwhich is portable, small, self-powered, lightweight, low-powered, andsafe to both the user and to the equipment being tested.

Another object of the present invention is to provide test equipment ofthe type described for testing circuits of low resistance, and fortesting circuits which have an unwanted resistance to ground of up toapproximately 75,000 ohms.

Many other advantages, features and additional objects of the presentinvention will become manifest to those versed in the art upon makingreference to the detailed description and the accompanying sheets ofdrawings in which a preferred structural embodiment incorporating theprinciples of the present invention is shown by way of illustrativeexample.

ON THE DRAWINGS FIG. l is a schematic diagram of the generator portionof a circuit testing device provided in accordance with the principlesof the present invention; and

FIG. 2 is an oscillogram of a signal provided by the circuit of FIG. 1.

AS SHOWN ON THE DRAWINGS The principles of the present invention areparticularly useful when embodied in a signal source such as shown inFIG. I, which is a circuit diagram of a signal generator which isconstructed to provide alternating current pulses or signals, andpreferably also to supply high voltage direct current. For the purposeof this invention, it is essential that the alternating current signalbe-other than sinusoidal, and to this end, a structure is provided bywhich there is produced a substantially square wave having a spike atthe leading edge of each pulse. The circuit in FIG. 1 shows arepresentative structure by which such a signal may be provided.Accordingly, there is provided a multivibrator 10 which includes adirect current powered pair of transistors ll, 12, a transformer 13, anda capacitor 14.

Each of the transistors 11 and 12 has an emitter 15 connected through anon-off switch 16 to the positive tenninal of a battery 17. Each of thetransistors 11 and 12 has a collector 18, such collectors 18 beingconnected together by a primary winding 20 of the transformer 13. Theprimary winding 20 includes a number of coils 21-24 connected in series.The collectors are also respectively connected to opposite sides of thecapacitor 14.

The primary winding 20 is center-tapped, and is connected by leads 25and 26, through a section 27 of a selector switch generally indicated at28, and through leads 29 and 30 t0 the negative terminal of the battery17.

The transistors 11 and 12 each have a base 31 respectively connected toone of a pair of resistors 32, 32, which are connected respectively toopposite ends of a center-tapped regenerative or tickler winding 33. Thewinding 33 serves as a control winding for regulating the bias potentialpresent on each of the bases 31, and comprises a pair of coils 34, 35.The center-tap of the winding 33 is connected by a lead 36 to a point ofreference, here indicated as being the negative terminal of the battery17.

When the switch 16 is first moved to a closed position, neither of thebases 31 is biased to block current flow therethrough. Thus, currenttends to flow from the positive terminal of the battery 17 through theswitch 16, through each of the emitters l5, and through each of thecollectors 18, the current flowing through the transistor 11 tending toflow through the coils 21 and 22 of the primary winding 20 to ground ornegative battery terminal in one direction, while the current flowingthrough the transistor l2 tending to flow through the coils 23 and 2d ofthe same winding to ground or negative battery terminal in the oppositedirection. This tendency creates a state of stability which isundesirable. To upset or unbalance such state of stability, there isprovided a low impedance capacitive connection from one end of theprimary winding 20 to the center-tap thereof, which is at the potentialof the negative terminal of the battery 17. To this end, a capacitor 37is provided which has one side thereof connected to the line whichconnects the coil 21 to the collector l8, and the other side of thecapacitor 37 is connected through a secondary winding 38, consisting ofa pair of coils 39 and 40, and thence through a pair of lines 41 and 42to the line 29, and hence through the line 30 to the negative terminalof the battery l7.

It will be appreciated that as the operation of transistor 11 isdependent upon induced voltages in winding 34, and operation oftransistor 12 is dependent upon induced voltages in the winding 35,maintenance of an induced control voltage on either transistor base isdependent upon the continuation of a current change in the operativeprimary winding 21-22 or 23 +24, and if such current reaches a limitingconstant value, for

any reason, induced control voltages in windings 34 and 35 will cease asa result of the absence of a flux change.

Such limiting current value may be derived in any suitable manner inaccordance with prior known multivibrator techniques, as for example, bytransistor saturation, transformer core saturation or by limiting thetransistor base driving power to a predetermined maximum value.

Consequently, upon closure of switch 16, voltage from the battery 17 isapplied to both transistor circuits and current will tend to flow inboth circuits. However, as a result of the presence of capacitor 37,current flow in the winding 2l-22 will be delayed by charging of suchcapacitor, while simultaneously with such switch closing current willimmediately flow in winding 23-24. Such current flow will produce aninduced voltage in each winding 35, 34 that in winding 35 having apolarity biasing the transistor 12 in a conductive direction and that inwinding 34 biasing transistor 11 in cutoff direction, whereby transistor12 will always dominate such initial operation.

The operation continues in this manner until the current flowing throughwinding 23-24, in dependence upon the circuit parameters, reaches saidlimiting value and resulting in the disappearance in the inducedvoltages in the windings 34 and 35. Upon discontinuance of the cutoffbias on the base of transistor 11, current will begin to flow in thewinding 2l22, resulting in the production of induced voltages in controlwindings 34, 35 but now of reverse polarity to the initial voltagestherein. induced voltage in winding 35 now promotes conduction oftransistor 11 and decreases that of transistor 12, such actioncontinuing until transistor -l2 is cutoff and the current of transistor11 reaches the limiting value, whereupon the operation will again bereversed.

The multivibrator thus has one astable or quasi-stable state wherein thetransistor 12 is conductive and the transistor 11 is blocking, and asecond astable or quasi-stable state wherein the transistor 11 isconducting and the transistor 12 is blocking. The primary windingtogether with the capacitor 14 comprise a circuit which, because of thecoaction of the tickler winding 33, switches back and forth betweenthese two astable states.

The other or third state of stability initially described is thusintermediate the astable states and is eliminated or upset by the lowimpedance capacitive connection including the capacitor 37 through thesecondary 38.

In this embodiment, there is a further low impedance capacitiveconnection which serves a similar function as the amount of delay willbe determined by the total effective shunt capacity creating the circuitasymmetry. To this end, a capacitor 43 has one side which is connectedto the same line as the capacitor 37, namely between the collector 18 ofthe transistor 11 and the coil 21. The capacitor 43 is connected at itsother side through a line 44 and through a further capacitor 45 and aline 46 to the line 30 and hence to the negative terminal of the battery17. Thus, a further current delaying path is provided to upset the stateof stability which otherwise would be present at the starting of themultivibrator 10.

The wave form of the signal induced at various points in the outputtransformer 13 is substantially that shown in FIG. 2, it beingunderstood that the voltages at such various points would differ, andthe height of the spike being determined by the size of the capacitor14.

A power outlet terminal is provided which includes a conductor 47connected through the lines 42, 29, and 30 to the negative terminal ofthe battery 17, and a conductor 48 which is connected to the movableelement of a further section 49 of the selector switch 28. Theillustrated position of the selector switch 28 represents an offposition thereof. The second termind thereof is connected by a line 50to the secondary winding 33 which is a high current, low voltagewinding, a typical potential being 0.2 volts. The next terminal to whichthe selector switch 28 may be positioned is connected through a line 51and a coupling capacitor 52 to the high voltage side of the coils 23, 24of the primary winding 20. A typical voltage available at this tap is 45volts. The fourth tap of the selector switch portion 49 is connected bya line 53 and a coupling capacitor 54 to the center-tap of a secondarywinding 55, composed of four coils 56-59. At this point, a potential ofvolts is typically available.

in use, the power outlet terminals 47, 48 are connected to the shortedcircuit to be tested, which may have a very low impedance, or whichmayhave a relatively high impedance, such as 75,000 ohms. The selectorswitch 28 is then positioned to approximately match the impedance of thesignal source with the line to be tested as described later herein. Anindication of strength of the battery 17 is provided by a voltmeter 55,the positive terminal of which is connected through a section 56a of theselector switch 28 and through a multiplier or range resistor 57a andthe switch 16 to the positive terminal of the battery 17. The negativeterminal of the voltmeter 55 is connected through the lines 29 and 30 tothe negative terminal of the battery 17. I 1

The output transformer 13 has laminations in the form of a hollowsquare. The coils 39, 21, 56, 34, 59, and 24 are disposed or wrappedabout one leg of such square, while the coils 40, 22, 57, 35, 58, and 23are wound about the opposite leg thereof. Each of the primary coils21-24 comprises 25 turns of two No. 29 wires wound and connected inparallel. Each of the secondary winding coils 56-59 comprises 900 turnsof No. 40 wire. Each of the coils 39, 40 of the secondary winding 38comprises three turns of No. 20 wire. The tickler winding 33 comprisesone center-tapped coil having 60 turns of No. 29 wire.

in addition to providing the nonsinusoidal periodic signal or pulse suchas indicated in FIG. 2, the generator may be utiliZBd to provide asource of high voltage direct current for conducting leakage tests. Tothis end, there is provided rectifier means indicated at 60, comprisinga fullwave voltage doubling rectifier circuit including four rectifiers61, the capacitor 45, and a further capacitor 62. One side of each ofthe capacitors 45 and 62 is connected by the line 44 to one side of thesecondary winding 55, and the other side of the secondary winding 55 isconnected by a line 63 to the opposite point in the bridge. The line 46,having a negative polarity from the battery 17, is connected between thecapacitor 45 and the adjacent rectifier, while a line 64 is connectedbetween the capacitor 62 and its adjacent rectifier 61. The line 64passes in series through a milliammeter 65 and thence to a fifth tap onthe selector switch section 49 for connection to the power outletterminal 48. Placing the selector switch 28 in this position thusdisconnects the transformer winding from the power outlet terrninal andconnects the rectifier means 60 thereto. Such positioning alsodisconnects the positive terminal of the voltmeter 55 from the rangeresistor 57a and connects it to a range resistor 66 of somewhat highervalue which is connected to the line 64.

When the generator is used as a source of AC pulses the center-tap ofthe primary winding 20 is connected through the second, third, andfourth taps of the selector switch section 27, and thence through thelines 29 and 30 to the negative terminal of the battery 17. When thegenerator is used as a source of high voltage direct current, with theselector switch in the fifth position, the foregoing connection for thecentertap is interrupted, and in place of the direct connection thusafforded between the battery 17 and the multivibrator 10, there isinserted in such center-tap line 25 an adjustable voltage divider 67 inthe nature of a potentiometer. One side of the potentiometer 67 isconnected through the switch 16 to the positive terminal of the battery17, and the other side of the potentiometer 67 is connected through aline 68, through the selector switch section 27 and the lines 29 and 311to the negative terminal of the battery 17. Thus, the entire batteryvoltage appears across the potentiometer winding. The potentiometerincludes a slider 69 connected through a momentary switch or push button70 to the line 25. With the selector switch in the fifth position, andwith the switch 116 closed, the multivibrator circuit is incompleteuntil the button 7% is depressed. Upon depressing such button, anddepending upon the position of the slider 69, a variable or adjustablepotential is applied to the multivibrator, by raising the potential ofthe center-tap of the primary winding with respect to the negativeterminal of the battery 17.

in AC operation of the device, the coils 58 and 59 float, but for DCoperation, a fourth section 71 of the selector switch 28 closes a pairof lines 72, 73 connecting the center-tap of the secondary winding 55 tothe coil 58, by which a relatively high voltage alternating current ismade available for rectification by the rectifier means 60.

The power outlet terminals &7 and 48 are connected to the shortedcircuit in such manner that current flows through the short therein.Assume that a transmission line or pair of conductors is disposed withina steel conduit, there being electrical continuity from the one to theother at a point comprising a short. If alternating current ofcommercial wave form and frequency is caused to flow in a completedcircuit including the pair of conductors, the magnetic field in the oneconductor is cancelled by the magnetic field in the other conductor. Soalso, when such current flows in the one conductor and in the conduitthrough the short, the field in one conductor of such paralleltransmission line is for all practical purposes can.- celled by thefield produced by the current in the other conductor or conduit of suchtransmission line. (The term transmission line" as used herein isemployed in a broad sense to include parallel wiring in general and itsconduit, cables, and the like, and should not be construed in arestrictive sense. Thus, the configuration and number of conductors mayvary considerably.)

However, in accordance with the principles of this invention, when theharmonically rich signal is applied from'the terminals a7 and 48 throughthe short, a somewhat different result occurs of which I take advantage.The velocity of propagation of a signal along a line is a function ofits frequency. Therefore, when many frequencies are present as in mysignal, individual frequency components do not all move at the samevelocity and do not all arrive at the short at the same instant.Therefore, there is a slight difference in magnetic field that makessubstantially complete cancellation thereof impossible. Statedotherwise, the nonsinusoidal waveform is produced by a large number ofharmonically related sinusoidal currents which are harmonics of thefundamental. These harmonics vary as to amplitude, as to frequency, andas to wavelength with respect to each other so that an external field isproduced by the net differences in current between these harmoniccomponents. By Fourier analysis, a wave form or shape may be broken downinto a fundamental sinusoidal frequency and a number of its sinusoidalharmonics. However, when they are combined as here, a nonsinusoidalresult is present. Where all such harmonics are integral harmonics ofthe fundamental, which is the situation here described, the outputsignal appears to be steady.

in the use of my pulse generator for detecting the location of a short,l prefer that at least one of the frequencies present in thenonsinusoidal waveform be one which is within the range of audiblefrequencies. This facilitates the use of earphones. More specifically, lprefer that the fundamental frequency be within such audible frequencyrange, and that it be above the highest ambient frequency present whichmight be expected from any other nearby source of magnetic fieldproduced by alternating current. In .many installations, it would thussuffice if the fundamental frequency were distinctly above 60 cycles persecond. in equipment where 400 cycles per second is customary, such asin locomotives or as in aircraft, it is therefore preferable that thefundamental be above such frequency, for example 500 cycles per second.The values given herein are those which I have employed to obtain aharmonically rich signal of the wave form shown in FIG. 2 having afundamental frequency of about 500 cycles per second.

Thus, in use, there is an application of a composite electricfrequencies in harmonic relation to each other, such composite currentbeing applied to the shorted circuit in such manner that the compositecurrent flows through the short itself. Doing so produces unbalancedcurrents which produce a net magnetic field which extends from theshorted circuit, even though the conductors thereof comprise a paralleltransmission line, coaxial cable, or a conductor within a conduit.Thereafter, using an appropriately sensitive detector (described in myaforesaid patent), the resulting magnetic field can be sensed andrelative measurements thereof made to indicate the physical limits ofsuch field, and hence the point where the field terminates. Such pointis physically defined by the short and is indicated by an abrupt changeby a large amount of the relative field strength. The steeper the slopeand the sharper the pip, tip, orspike on the waveform the more harmonicsare present and the more effective the instrument becomes for mypurpose.

The strongest signal sensed by the detector for each of the AC settingsof the selector switch 28 indicates the most power transfer, and hencethe best matching of the impedance of the signal source with the line tobe tested. a

Preferably, the maximum voltages available at the outlet terminals 47,48 are indicated adjacent to the selector switch 28 so that the useriswarned against possibly using the same in conjunction with circuitcomponents that might be damaged by application of such a, potential.

when the selector switch 28 is in the DC position, the slider 69 ispreferably first moved to a minimum voltage position and then the button70 is depressed. Thereafter, the slider 69 is moved so as to graduallyincrease the DC voltage and the current to any conveiiient levelindicated on the meters 55 and 65 from which the DC resistance can becalculated according to Ohms law. DC voltages up to 350 volts are thusavailable with the circuit described.

The following values have been utilized:

l l, 12 Tung Sol ET6 Transistors 17 Eveready 409 (6V) Battery 61 SarkesTarzian M500 or Pacific Semiconductor TP400 Diodes 65 1,000 ohms pervolt Meter Although various minor modifications might be suggested bythose versed in the art, it should be understood that I wish to embodywithin the scope of the patent warranted hereon all such embodiments asreasonably and properly come within the scope of my contribution to theart.

lclaim: 1. An AC pulse generator for providing fault-detection signals,comprising:

a. a multivibrator circuit; b. an output transformer having acenter-tapped winding forming a part of said multivibrator circuit and asecondary winding connected to output terminal means; and v c. at leastone capacitive connection extending through said secondary winding fromone end of said first-mentioned winding to its center-tap, providing acapacitive shunt across such section of this winding to render themultivibrator circuit asymmetrical.

2. An AC pulse generator according to claim 1, wherein saidmultivibrator circuit is of the astable type and having two currentcomposed of sinusoidal alternating currents of quasi-stable statesnormally operative to switch periodically from one of said states to theother, and having a third state of stability intermediate saidquasi-stable states, said third state being upset by said capacitiveconnection.

3. An AC pulse generator according to claim 1, comprising in furthercombination a capacitor connected across said center tapped windingwhereby said multivibrator circuit has an alternating output ofnonsinusoidal wave form with at least one pip per cycle.

4. An AC pulse generator according to claim 1, having at least oneadditional secondary winding in said transformer; and a multipositionselector switch connected between said windings and said output terminalmeans.

5. An AC pulse generator according to claim 4, including a battery fordirectly driving said multivibrator circuit; and said selector switchbeing of the ganged type having a plurality of positions for connectingsaid battery to said multivibrator circuit to power it while alsoconnecting a selected one of said windings of appropriate impedance tosaid output terminal means.

6. An AC pulse generator according to claim 1, wherein said transformerhas a secondary winding for providing a relatively high voltage, andrectifier means including capacitors connected to the last-mentionedsecondary winding for providing high voltage direct current to saidoutput terminal means.

7. An AC pulse generator according to claim 6, wherein an additionalcapacitive connection is provided between one end of said primarywinding and its center-tap, which capacitive connection includes one ofsaid capacitors of said rectifier means.

1. An AC pulse generator for providing fault-detection signals,comprising: a. a multivibrator circuit; b. an output transformer havinga center-tapped winding forming a part of said multivibrator circuit anda secondary winding connected to output terminal means; and c. at leastone capacitive connection extending through said secondary winding fromone end of said first-mentioned winding to its center-tap, providing acapacitive shunt across such section of this winding to render themultivibrator circuit asymmetrical.
 2. An AC pulse generator accordingto claim 1, wherein said multivibrator circuit is of the astable typeand having two quasi-stable states normally operative to switchperiodically from one of said states to the other, and having a thirdstate of stability intermediate said quasi-stable states, said thirdstate being upset by said capacitive connection.
 3. An AC pulsegenerator according to claim 1, comprising in further combination acapacitor connected across said center tapped winding whereby saidmultivibrator circuit has an alternating output of nonsinusoidal waveform with at least one pip per cycle.
 4. An AC pulse generator accordingto claim 1, having at least one additional secondary winding in saidtransformer; and a multiposition selector switch connected between saidwindings and said output terminal means.
 5. An AC pulse generatoraccording to claim 4, including a battery for directly driving saidmultivibrator circuit; and said selector switch being of the ganged typehaving a plurality of positions for connecting said battery to saidmultivibrator circuit to power it while also connecting a selected oneof said windings of appropriate impedance to said output terminal means.6. An AC pulse generator according to claim 1, wherein said transformerhas a secondary winding for providing a relatively high voltage, andrectifier means including capacitors connected to the last-mentionedsecondary winding for providing high voltage direct current to saidoutput terminal means.
 7. An AC pulse generator according to claim 6,wherein an additional capacitive connection is provided between one endof said primary winding and its center-tap, which capacitive connectionincludes one of said capacitors of said rectifier means.